Most copying systems employ a fixed focus lens. Should it be desired to vary the magnification of the lens, it is necessary that the lens be suitably moved so that the relative distance between the object and image surfaces is varied. The present invention relates however to a lens system in which the distance between the object and image surfaces is maintained constant and the lens system is moved therebetween enabling variation copying.
Prior art copying lens systems may be classified into two groups, one group having a wide range of magnification extending from a minimum reduced size to a maximum enlarged size and the other having a narrow range of magnification extending only from the reduced size to the size of the original or from the size of the original to the enlarged size. A lens system of the first type is disclosed in unexamined Japanese Patent Publication Nos. 49/1242 and 53/60655 and Japanese Patent Application 53/84872 which are assigned to the assignee of the present application. In these lens systems, the magnification variation ratio is high resulting in the lens system having a large physical size. Such a lens system moreover requires three or four separate lens groups with the result that the movements of the lens groups are intricate for maintaining constant the distance between the object surface and the film surface.
The latter case is disclosed in unexamined Japanese Patent Publication No. 54/44554 which relates to a simple lens having a small magnification variation ratio of less than 2. This lens is composed of two lens groups in which the refractive power of the first lens group is extremely small and is fixed at one end of the variational range. In spite of a very small magnification variation ratio of about 1.4, the variation of the overall focal length is extremely small. For this reason, when a magnification value in the range from the minimum reduced value to equal magnification value is used, the first lens group must have a negative refractive power which simultaneously the distance between the first and second lens groups must be lengthened thereby disadvantageously increasing the size of the lens system. When a magnification value in the range from the point of equal magnification to the maximum enlarged value is used, the first lens group must have a positive refractive power while for equal magnification and the distance between the lens group must also be increased thereby again increasing the size of the lens system.
It is an object of the present invention to facilitate the photographic operation of a camera such as a large size printing camera and a duplicating camera by fixing the picture base and the image surface. Also it is another object to provide a camera used in an optical system for a copying machine in which the copy size can be varied. Particularly, in such a copying lens, it is desirable that various aberrations be well corrected for and that there be provided a sufficient quantity of marginal light.
A yet further object of the present invention is to provide a practical and useful zoom lens system for copying with the maximum magnification variation being below a value of approximately two. It is also an object of the invention to provide such a lens system which is very simple in construction and which is composed of two lens groups. It is further an object of the invention to provide such a lens system in which the negative refractive power of the first lens group and the positive refractive power of the second lens group are both stressed so that the overall system may be miniaturized and, in spite of the resulting strong asymmetrical pattern of the lens system, various aberrations, particularly distortion, are well corrected for.